This invention relates to electronic telephone networks and, more particularly, to a network which adapts itself to a wide range of telephone line lengths, losses, and operating conditions.
Conventional telephone networks include a carbon microphone, a transformer hybrid, a dynamic receiver, and various components such as resistors, capacitors, etc., which are used to interface a standard two-wire telephone line with the telephone handset. Due to the passive nature of the primary components (e.g., the carbon microphone) the standard telephone networks have several disadvantages such as large physical size, lack of fidelity, compatibility with other electronic telephone devices, and versatility. Electronic telephone networks are also known in the art and obviate many of the above enumerated disadvantages while providing transmit and receive gain and power output sufficient to enable the use of low-efficiency dynamic transducers in the handsets. However, these electronic telephone networks have several known disadvantages in that they are expensive, fragile and, more importantly, incompatible with many existing telephone facilities or practices. For example, the incompatibility problem arises due to the relatively high DC voltage requirement of the electronic networks as they derive their power from the telephone line. Disadvantageously, this prevents or severely limits parallel operation with conventional telephone networks. Furthermore, the high DC resistance of these prior art networks limits the maximum telephone line loop lengths on exchanges with conventional supervision equipment which require certain minimum current drain levels for satisfactory operation.
These and other disadvantages are overcome by the present invention wherein an electronic telephone network is provided which provides the normal functions of an electronic telephone network but which further provides: an electronic hybrid to separate the receive signal from the transmit signal while eliminating objectionable sidetone; correct DC line current drain levels for telephone central office supervision equipment; relatively constant receive and transmit levels over the entire range of conventional telephone line distances between the network and the exchanges; efficient use of available DC power; independence of the AC impedance characteristics of the telephone line; and, more particularly, while providing operation for DC terminal voltage between the normal maximum of 8.0 volts down to, and including, 2.2 volts.